1. Field of the Invention
This invention relates to a multicolor image forming apparatus and more particularly to an improved multicolor image forming apparatus for obtaining a multicolor image by successively forming images of different colors on an image retainer based on image data and suitable for use in electrostatic recording or electrophotography. More specifically, this invention relates to apparatus and method for controlling color dot size in a multicolor image.
2. Description of the Prior Art
Heretofore, multicolor images have been formed, for example, by means of electrophotography, wherein a copying process, including charging, exposing, developing and transferring steps, in is repeated on a component color basis and toner images of various colors are superposed on copying paper. Latent images provided by the above-described steps are formed using the separated light of blue, green, red, etc. obtained from a color separation filter and toner images are formed by developing the latent images with yellow (Y), magenta (M), cyan (C) and, if necessary, black (K) toners and then the toner images are transferred to and superposed on recording paper to form a multicolor image. However, such a multicolor image forming method is disadvantageous in that .circle.1 the apparatus becomes large-sized and the time required for image formation is lengthy because toner images must be transferred to a transfer member each time the color development is completed; and .circle.2 registering accuracy must be guaranteed because image transfer is repeated.
There has been proposed a multicolor image forming method intended to solve the above-described problems by developing a plurality of toner images superposed on one and the same photosensitive member so that the transfer process may be completed at one time. (The expression "superposition of toner images" as used herein means not only a case where toner particle layers forming an image are physically superposed but also another case where they are not physically superposed but toner separately adheres to the photosensitive member in different positions, and the same definition will apply to the following) description. Nevertheless, bad effects are still brought about when using that method and result from disturbing the toner image obtained from development in the preceding stage or from upsetting color balance in a multicolor image when the toner in the developer in the preceding stage mixes with that in the following stage.
The method proposed to avoid such disadvantages comprises applying a bias voltage with an a.c. component superposed thereon to a developing device during and after the second development and flying toner onto the electrostatic image formed on a photosensitive member to form a multicolor image. Since the layer of a developer is prevented from rubbing the toner images formed up to the preceding stage in that method, no disturbance of the image occurs.
Referring to the flowchart of FIG. 18, the principle of the prior art image forming method will now be described. FIG. 18 shows changes in potential on the surface of a photosensitive member, taking a case where the charging electrode polarity is positive as an example. In FIG. 18, there are shown the exposed portion PH of a photosensitive member, the unexposed portion DA of the photosensitive member and an increase DUP in electric potential due to the attaching of positively charged toner to the exposed portion PH derived from the first developing.
The photosensitive member is uniformly charged by a scorotron charger and the surface thereof is at a fixed positive surface potential E as shown in FIG. 18(a). Subsequently, a first image exposure is applied with a laser, CRT, LED, or a liquid crystal shutter as an exposure source and the potential of the exposed portion PH decreases in proportion to the quantity of light as shown in FIG. 18(b). The electrostatic latent image thus formed is developed by a developing device and carries the positive bias applied thereto, the positive bias being roughly equal to the surface potential E of the unexposed portion thereof. Consequently, the positively charged toner T is allowed to adhere to the exposed portion PH at a relatively low potential as shown in FIG. 18(c) and the first toner image T is formed. Although the potential of the region where the toner image T has been formed rises by DUP because of the positively charged toner T adhering thereto, it does not become equipotential to that of the unexposed portion DA. The surface of the photosensitive member provided with the first toner image formed thereon is subsequently charged again by the charger and remains at a uniform surface potential E, irrespective of the presence of the toner T, which is illustrated in FIG. 18(d). A second image exposure is applied to the surface of the photosensitive member to form an electrostatic latent image FIG. 18(e) and, as in the case of FIG. 18( c), a positively charged toner image T' different in color from the toner T is developed to obtain a second toner image, which is shown in FIG. 18(f). The process above described is repeated to obtain a multicolor toner image on the photosensitive member. The image is then transferred to recording paper and heated or pressurized for fixing and a multicolor recording image is obtained. In that case, the toner and charge remaining on the surface of the photosensitive member are removed and cleaned and used for a subsequent multicolor image formation. On the other hand, there is another method of fixing a toner image on the photosensitive member.
In the method described in FIG. 18, at least the developing step of FIG. 18(f) should preferably be implemented without allowing the developer layer to contact the surface of the photosensitive member.
In the above-described method of multicolor image formation, the second and following charging may be omitted. When charging is repeated each time without such omission, a charge eliminating step may be added before charging by means of a lamp or corona discharge. Moreover, the exposure light source used for each image exposure may be either the same or different.
In the above-described method of multicolor image formatio, toners of four colors, i.e., yellow, magenta, cyan and black are often superposed for the following reason: A black image ought to be obtained by superposing the three primary colors of yellow, magenta and cyan but, because the toners for the three primary colors do not, as a practical matter, have an ideal absorption spectrum wavelength region, the three color mixture will not become completely black, whereby the density in a color image tends to be insufficient. Moreover, because of the incorrect registering of toner images of three primary colors, the toner of three primary colors alone is incapable of reproducing the clear black color required for characters and lines. In order to solve that problem, in addition to the three primary colors, black is also used to form a multicolor image as aforementioned.
In addition to the above-described electrophotography method of forming a latent image for forming a multicolor image, there are other methods of directly forming a electrostatic latent image on an image retainer using a multi-stylus electrode, a screen photosensitive member or control electrode and forming a magnetic latent image using a magnetic head. A recording apparatus utilizing the direct method is incorporated into such a system as shown in the block diagram of FIG. 19. In this example, a reader with a solid pickup element is used to read an original having multicolor optical data and the image data obtained is converted by an image processor into what is fit for the recording apparatus (hereinafter refer to as "recorded data").
There are two methods for expressing various colors according to the above-mentioned methods:
(1) A method of not directly superposing toners of different colors (FIGS. 20A, 20B); and
(2) A method of superposing toners of different colors (FIG. 20C).
FIGS 20A through 20C illustrate an arrangement of toner colors written to an image retainer. As shown in FIG. 21A, in a first method, toners T.sub.1, T.sub.2 are distributed without being superposed to reproduce color artificially on recording paper. As shown in FIGS. 22 and 23, in a second method, toner of a certain color is superposed on a toner image of different color and developed to reproduce color.
However, in the case of electrophotography, for instance, because the color in the second method above is absorbed in the second the toner T previously developed and is unable to reach the photosensitive layer of the image forming body satisfactorily, a latent image is not completely formed. Consequently, the adhered quantity of toner T.sub.2 developed later tends to decrease as shown in either FIG. 22 or FIG. 23. In the first method above, the registering of image exposure must be carried out strictly so that the toner image of one color is not exposed to the toner image of another color at the same position. If the image exposure is inaccurately positioned as shown in FIG. 21B, the toner image T.sub.1 in the preceding stage will intercept part of the image exposure and the adhering quantity of the toner image T.sub.2 developed in the following stage will tend to decrease as shown in FIG. 21C. That trend indicates that recording characteristics will differ according to the spectral sensitivity of an image retainer, the spectral characteristics of a light source for use in image exposure, the spectral transmittance characterisics of toner and the order of colors being developed.
On the other hand, although each of these latent image forming methods is capable of expressing gradation by means of multi-value recording, the expression of gradation through those methods requires a large capacity of image data because they rely on so-called multistage gradation. To provide high-speed stable recording with a small capacity of image data, accordingly, there has been proposed a method for expressing gradation artifically by converting each inputted picture element into a binary value. There are known, for example, the density pattern method of FIG. 24 and the dither method of FIG. 25 for toner of each color.
The density pattern method of FIG. 24 comprises converting a picture element having inputted gradation into one that has a plurality of binary gradations. In FIG. 24, there are shown an input image 1a, a sample 2a for taking a picture element 5a having a representative density value of a matrix of the above-described input image 1a and processing the picture element 5a, and M.times.N reference density matrix 3a for converting the sample into a binary value and a pattern 4a obtained as the result of which the sample 2a above is compared with the reference density matrix 3a and converted into a binary value.
The dither method of FIG. 25 is used to convert a picture element having the gradation of an input image into one that has binary gradation. In FIG. 25, there are shown an input image 1b, a sample 2b representing a particular M.times.N picture element matrix of the input image 1b, the sample 2b being used for the binary conversion process, a reference density M.times.N matrix 3b for converting the sample into a binary value and a pattern 4b obtained as the result of which the sample 2b is compared with the reference density matrix 3b and converted into a binary value. Such a pattern is arranged for each color of FIGS. 20A through 20C.
In the conventional multicolor image forming apparatus, the data obtained by subjecting the color image data received to color separation is recorded by comparing the data with a reference signal read out of a memory and converting it into a binary value. The following discriminating process, for instance, is applicable:
(1) A special discriminating sensor is used to read an original;
(2) When the area of a background is large, the original is judged as a line drawing, whereas it is judged as a gradation drawing when the area is small.
(3) When it is a line drawing, it is converted into a binary value as a dither matrix;
(4) Each image data on yellow, magenta, cyan and black is compared with a reference value and then converted into a binary value;
(5) The number of picture elements reaching the H level according to process step (4) is counted on a color basis and they are classified into Yo, Mo, Co, Ko;
(6) Judgement is made if there is a particularly large one among Yo, Mo, Co.;
(7) If the difference is large as the result of process step (6), it will be judged as a single color image and the predetermined dither matrix is applied to each color;
(8) If the difference of each color is small as the result of process step (6), it will be judged as a multicolor image and the predetermined dither matrix is applied to each color; and
(9) The image data is processed.
If a multicolor image is formed based on the above-described conditions, any desired color will become reproducible and a recorded image fit for each kind of input image data will be obtainable.
If the image density and color reproducibility are specified by automatic control or by apparatus effecting operation externally, recording characteristics may be caused to change by controlling recording conditions such as the developing bias or charge potential of the recording apparatus when each toner image is formed. However, it is hardly possible by control of recording conditions alone to obtain the desired recording characteristics and completely prevent character jumping or fogging.